1. Field of the Invention
This invention concerns an apparatus and process for gripping and withdrawing the fuel rods out of a spent nuclear fuel assembly.
2. Description of the Prior Art
Gripping devices for removing the fuel rods from spent nuclear fuel assemblies are known in the prior art. The purpose of these devices is to reduce the storage space needed to store spent fuel rod assemblies in the storage pool of a nuclear power plant. In such assemblies, the fuel rods are vertically oriented and rectangularly spaced from one another by spacing grids. Such gripping devices grip and slidably withdraw the rectangularly spaced fuel rods out of the grids of their respective fuel rod assemblies so that they may be closely packed together in the on-site storage pool, thereby substantially reducing the storage space requirements for the spent assembly.
In one such prior art fuel rod gripper, the fuel rods are grippingly engaged by a plurality of cylindrical collets which are spring-loaded into the tapered holes of a locking plate. In this mechanism, the holes are tapered in both directions so that they flare out both at the bottom and the top of the locking plate. The cylindrical collets are inserted into the top tapered portions of these holes. The bottom tapered portion of these holes facilitates the insertion of the top end of a fuel rod into the hole. The gripper is lowered far enough into an array of spent fuel rods so that the ends of the rods extend completely through the tapered holes and into the hollow interior of the cylindrical collets. When the springs associated with each of the collets are compressed, the collets are forced into the upper tapered portion of their respective holes, which in turn causes them to contract around the ends of their respective fuel rods in gripping engagement. This particular fuel rod gripper is more specifically disclosed in U.S. patent application Ser. No. 418,141, filed Sept. 19, 1982, and assigned to Westinghouse Electric Corporation, now U.S. Pat. No. 4,551,299.
While the aforementioned fuel rod gripper has been successfully used to remove fuel rods from a spent fuel assembly, it is not without shortcomings. For example, the use of mechanical springs to apply a biasing force onto each of the collets causes the collets to apply a non-uniform gripping force to their respective fuel rods which is proportional to the diameters of their respective rods. Since there is normally some variation in the diameters of the rods, this prior art gripper applies non-uniform gripping forces on the rods. Such non-uniform gripping forces adversely affect the reliability of this gripper; i.e., some rods may be very firmly gripped, while others are not. Additionally, there is no provision in this gripper to conveniently confirm whether or not each of the collets has in fact grippingly engaged its respective fuel rod. This design deficiency can result in substantial delays in the operation of the device; when the gripper fails to grip all of the rods, it may have to be completely re-lowered and re-engaged with the removing rods. Further, such gripping failure necessitates a time-consuming "off-normal" operating mode in which the spent rods are transferred to an intermediate location before being deposited in their final location in the storage pool. Still another deficiency in the design of this gripper is the difficulty of replacing broken or damaged collets. The spring mechanism to which each collet was connected makes it difficult, if not impossible, to replace a single broken collet without completely disassembling this gripper. Finally, because the gripping action of each of the collets is much like a "Chinese handcuff" (i.e., the gripping force becomes greater in proportion to the resistance it encounters in lifting the fuel rod), the collets of this prior art gripper can inadvertently crush or tear the ends of their respective fuel rods if the rods become bound in the grids of the fuel rod assembly. Such breaking of the integrity of the rods can release radioactive uranium dioxide directly into the water of the core area of the reactor, thereby contaminating it.
Clearly, a need exists for a multiple fuel rod gripper which is capable of applying a uniform gripping force onto each one of the fuel rods of a spent fuel rod assembly, and which includes a means for confirming whether or not each of its collets has grippingly engaged its respective rods. Additionally, it would be desirable if such a gripper applied a limited maximum gripping force onto the ends of the fuel rods so that the rods will not become crushed or broken in the event they become stuck in the grids of the fuel rod assembly. Finally, it would be desirable if the collets could be individually replaced without the necessity of disassembling the entire gripper device.